Gas-fluidized fine powders display three regimes of fluidization: solidlike, fluidlike, and bubbling. We investigate, from both macroscopic and local measurements, the transition between these regimes. We show that the transition between the solidlike and the fluidlike regimes takes place along an interval of gas velocities in which transient active regions alternate with transient solid networks. Although in the apparently homogeneous fluidlike regime large amplitude bubbles are not perceived and the bed expands continuously with increasing gas flow, optical probe local measurements show the existence of mesoscale pseudoturbulent structures and short-lived voids, reminiscent of liquid-fluidized beds behavior, and whose characteristic temporal frequency increases with gas velocity. These mesostructures might be responsible for the fast diffusion measured in gas-fluidized beds.

We present measurements on the settling velocity of gas-fluidized beds of fine cohesive powders. In the solidlike regime ͑solid volume fraction Ͼ c ) particles are static, sustained by enduring contacts. The settling is hindered by interparticle contacts and is a very slow process. In the fluidlike regime (Ͻ c ) permanent contacts no longer exist, and the bed displays a diffusive dynamics. The interparticle adhesive force leads to the formation of particle aggregates, and for this reason the sedimentation velocity exceeds the predicted value by empirical or theoretical laws on the settling of individual particles. We use an extension of the Richardson-Zaki empirical law for the settling of aggregates in the fluidlike regime to fit the experimental data. Aggregates are characterized by the number of aggregated particles N and by an effective radius R. The trend followed by these parameters with particle size is confirmed by direct visualization of the aggregates, and shows that cohesive effects become important when the adhesion force between particles is above particle weight. Results show that aggregates form open structures with a fractal dimension close to the predicted one in the diffusionlimited-aggregation model (Dϭ2.5).

We report a novel experimental study on the jamming transition of dry fine powders with controlled attractive energy and particle size. Like in attractive colloids dry fine particles experience diffusionlimited clustering in the fluidlike regime. At the jamming threshold fractal clusters crowd in a metastable state at volume fractions depending on attractive energy and close to the volume fraction of hard nonattractive spheres at jamming. Near the phase transition the stress-(volume fraction) relationship can be fitted to a critical-like functional form for a small range of applied stresses ÿ J as measured on foams, emulsions, and colloidal systems and predicted by numerical simulations on hard spheres. DOI: 10.1103/PhysRevLett.92.258303 PACS numbers: 83.80.Fg, 81.20.Ev, 47.55.Kf Supercooled liquids, granular systems, and colloidal suspensions are systems that display a nonequilibrium kinetic transition from a fluidlike to a solidlike jammed regime [1]. At jamming the constituent particles are suddenly arrested in a metastable static state forming a solid disordered network that spans the system. The jamming transition has been described by a phase diagram parametrized by interparticle attractive energy U, temperature T, particle volume fraction , and applied stress [2,3]. For example, granular systems jam when they are compressed or shear stress is lowered, a liquid jams when it is cooled, and colloid particles gelate with increasing U. Light scattering experiments suggest a link between the jamming transitions for suspensions of hard (nonattractive) spheres (U=K B T 0, where K B T is the thermal energy) and for suspensions of attractive particles (U=K B T > 0) [4]. While the kinetic arrest is driven by crowding of single particles in the absence of attractive forces, for attractive suspended particles jamming is driven by the crowding of fractal clusters. Suspensions of nonattractive hard spheres jam at J 0:56-0:59, which is comparable to the random loose packing (RLP) of nonattractive hard spheres at the limit of zero gravitational force ( RLP ' 0:56) but is well below the random close packing (RCP) limit ( RCP ' 0:64) [5]. On the other side, jamming of strongly attractive suspended particles takes place at J J U . In the limit U=k B T 1 fractal clusters crowd by a diffusion-limited cluster-cluster aggregation process (DLCA [6]). Since the density of this fractal structure decreases as it grows the system can form a gel at arbitrary small ( J

Gas-fluidized powders of fine particles display a fluidlike regime in which the bed does not have a yield strength, it expands uniformly as the gas velocity is increased and macroscopic bubbles are absent. In this paper we test the extension of this fluidlike regime as a function of particle size and interparticle attractive force. Our results show that for sufficiently large particles, bubbling initiates just after the solidlike fluidized regime as it is obtained experimentally by other workers. A scaling behavior of the solid-phase pressure in the fluidlike regime and a predictive criterion for the onset of macroscopic bubbling are analyzed in the light of these results.

Fluidized fractal clusters of fine particles display critical-like dynamics at the jamming transition, characterized by a power law relating consolidation stress with volume fraction increment [^c / ]. At a critical stress clusters are disrupted and there is a crossover to a logarithmic law ( log^c) resembling the phenomenology of soils. We measure ÿ@ 1= =@ log^c / Bo 0:2 g , where Bo g is the ratio of interparticle attractive force (in the fluidlike regime) to particle weight. This law suggests that compaction is ruled by the internal packing structure of the jammed clusters at nearly zero consolidation. DOI: 10.1103/PhysRevLett.94.075501 PACS numbers: 61.43.Gt, 45.70.Cc, 61.43.Hv, 81.20.Ev Empirical studies on the compaction of soils date back to the beginning of the last century. Walker [1] fitted his data by the logarithmic law 1= ÿ log c = c0 , where is the particle volume fraction, c the applied consolidation stress, and (compression index) and c0 are empirical parameters. This equation applies well in loose samples, where compaction is driven by rearrangement of particles, and has been traditionally used in civil engineering [2,3]. An essential ingredient in most granular systems is cohesion. Tests on cohesive powders show that decreases with the particle volume fraction of the initial state [4,5], indicating that interparticle attractive forces, which favor the formation of porous structures, play a relevant role in the compaction process. Yet the initial state in typical engineering experiments involves consolidation stresses c0 > 10 kPa [5]. Many industry applications demand research on smaller consolidations as these correspond to conditions of powder flow. For example, in the handling of xerographic toners, typical consolidations range from a few pascals to a few hundred pascals. Moreover, experiments at low consolidations have a fundamental interest in order to characterize the transition from the fluidlike to the solidlike state (jamming) [6] since the structural properties of the unconsolidated jammed state ( c ' 0), which is the truly initial state in any compaction process, are determinant on the rearrangement of the further loaded particles. We study the compaction of fine particles with controlled attractive force, initially fluidized and later subjected to loads from just a few pascals up to 10 kPa. Our novel experimental study is aimed to shed light on the role of the initial state, i.e., the unconsolidated jammed state, on compaction. The powders tested are xerographic toners based on polymer (particle density p ' 1 g=cm 3 ). They are produced by an attrition process, thus having an irregular shape, and size classified in a range of particle sizes (d p ) from 19.1 to 7 m by aerodynamic classification, showing a narrow particle size distribution (see Fig. 1). Additionally, the powders are blended with fumed silica nanoparticles (either 8 or 40 nm nominal diameters) to coat uniformly the polymer particle surface in concentrations from 10% to 100% of surface area coverage (SAC).In the fl...